Allow macros to take self as an argument

[PizzasBear]

Fixes the error no field `self` on type `&Templ` in the following example

#[derive(askama::Template)]
#[template(
    ext = "html",
    source = "
        {% macro m(x) %}
            {{ x.name }}
        {% endmacro %}

        {# this doesn't work #}
        {% call m(self) %}
    "
)]
struct Templ {
    name: String,
}

fn main() {
    println!(
        "{}",
        Templ {
            name: "Templ".into()
        }
    );
}

[PizzasBear]

A temporary fix is to wrap self in parentheses, e.g.

#[derive(askama::Template)]
#[template(
    ext = "html",
    source = "
        {% macro m(x) %}
            {{ x.name }}
        {% endmacro %}

        {# this does work #}
        {% call m((self)) %}
    "
)]
struct Templ {
    name: String,
}

fn main() {
    println!(
        "{}",
        Templ {
            name: "Templ".into()
        }
    );
}

[djc]

Seems okay -- since we know self is a reference, we know we won't be moving a non-Copy value.

Could you maybe add a test for this?

[PizzasBear]

I'm pretty new to pull requests and this repo, so I don't know how to add a test...

[djc]

Basically add your example from the PR description in testing/tests/macro.rs and add a test function to make sure it has the expected output.

[PizzasBear]

I've added the test

[djc]

Thanks!